1. Field of the Invention
The present invention relates to a piezoelectric actuator comprising a piezoelectric layer that is formed by crystallizing a ferroelectric material, and more particularly relates to a piezoelectric actuator which comprises strontium ruthenate in the bottom electrode, and PMN-PT in the piezoelectric layer.
2. Description of the Related Art
Piezoelectric actuators are constructed by sandwiching a piezoelectric layer that has an electro-mechanical transducing capacity between two electrodes. Electrostrictive ceramics and piezoelectric ceramics are known as material used in such a piezoelectric layer. In order to improve the characteristics of piezoelectric actuators, research has been pursued both from the standpoint of the ceramic material that forms the piezoelectric layer and the bottom electrode material that forms the substrate on which the piezoelectric layer is crystallized.
Composite oxides which have a perovskite crystal structure, and which can be expressed by the chemical formula ABO3 are known as piezoelectric ceramics. For examples lead zirconate titanate (PZT) in which lead (Pb) is used as A, and a mixture of zirconium (Zr) and titanium (Ti) is used as B, is well known. Besides PZT, there is also barium titanate (BaTiO3) which is advantageous in that no lead is used, and which also offers the advantages of a large amount of displacement, inexpensive raw materials and an extensive record of use.
Furthermore, relaxor ferroelectric materials have attracted attention as materials that have a large electro-mechanical coupling factor and piezoelectric coefficient, and that can be used in place of PZT type ceramics. In regard to relaxor ferroelectric materials, for example, PMN-PT (Pb(Mg1/3Nb2/3)O3—PbTiO3) and PZN-PT (Pb(Zn1/3Nb2/3)O3—PbTiO3) may be cited as examples of important materials.
Traditionally, metal materials such as Pt or the like have been used as electrode materials. In recent years, however, strontium ruthenate (SrRuO3 or the like) has attracted attention. The reason for this is that since strontium ruthenate has a perovskite crystal structure similar to that of PZT type piezoelectric layers, the joining characteristics at the interface are superior so that the epitaxial growth of PZT or the like can easily be realized. Furthermore, this material is also superior in terms of characteristics as a barrier layer that prevents the diffusion of Pb in the piezoelectric layer.
By combining a piezoelectric layer consisting of PZT with electrodes consisting of strontium ruthenate, it is possible to provide a piezoelectric actuator that has good characteristics. For example, a composite electrode structure using a buffer layer in a case where a dielectric layer consisting of PZT is formed on a bottom electrode consisting of strontium ruthenate is disclosed in Japanese Patent Application Laid-Open No. 2001-122698.
However, it is desirable that metal oxide electrodes consisting of strontium ruthenate or the like have lattice constants that match those of the ferroelectric material of, the piezoelectric layer that directly contacts such electrodes, and that the adhesion between the silicon substrate and the piezoelectric layer be good. The material of the buffer layer and the ferroelectric material of the piezoelectric layer cannot be arbitrarily combined. In concrete terms, the crystalline state of the oxide such a strontium ruthenate or the like that forms the bottom electrode must be controlled so that this structure matches the lattice constants of the ferroelectric material, and at the same time, this oxide must have chemical properties which are such that tight adhesion between the silicon substrate and bottom electrode is ensured.
What has been studied experimentally in the past is the structure of metal oxide electrodes in cases where PZT is used as the material of the piezoelectric layer. In cases where ferroelectric materials other than PZT have been used, it has not been clear what type of layer structure or crystalline state is required in such metal oxide electrodes. It may be predicted that good piezoelectric actuators can be provided in cases where relaxor ferroelectric materials such as PMN-PT, PZN-PT or the like (which have especially superior piezoelectric characteristics) are used together with metal oxide electrodes consisting of barium titanate or the like (which are desirable from the standpoint of protection of the environment).